The invention pertains to apparatus and methods for non-invasively and quantitatively evaluating the status of bone tissue in vivo, as manifested, at a given time, through one or more of the quantities: bone-mineral density, strength, and fracture risk.
In recent years, various attempts have been made to use ultrasonic energy to assess the condition of bone tissue, in vivo, but these attempts have been essentially ad hoc, with no consistent framework within which to analyze data. A great deal of information is obtainable from ultrasonic experiments, but much of the information has not been used. The signal-processing techniques that have been used have been so simple as to ignore available and useful aspects of the data, and the signal-to-noise ratio of experimental data has been relatively poor.
U.S. Pat. No. 3,847,141 to Hoop discloses a device to measure bone density as a means of monitoring calcium content of the involved bone. A pair of opposed ultrasonic transducers is applied to opposite sides of a patient's finger, such that recurrent pulses transmitted via one transuducer are "focused" on the bone, while the receiving response of the other transducer is similarly "focused" to receive pulses that have been transmitted through the bone. The circuitry is arranged such that filtered reception of one pulse triggers the next pulse transmission; the filtering is by way of a bandpass filter, passing components of received signals, only in the 25 to 125 kHz range; and the observed frequency of retriggering is said to be proportional to the calcium content of the bone. Thus, Hoop is not concerned with anything more than what he perceives to be transit time for pulses in the indicated band.
Pratt, Jr. is identified with a number of U.S. patents, including Nos. 4,361,154, 4,421,119 (divisionally related to the '154 patent, and subsequently reissued, as Re. No. 32,782), 4,913,157, and 4,941,474, all dealing with establishing, in vivo, the strength of bone in a live being such as a horse. In the first three of his patents, the inventor bases his disclosures on the measurement of transit time from "launch" to "reception" of pulses of 0.5 MHz and 1.0 MHz through the bone and soft tissue, and from measurement of pulse-echo time, to thereby derive a measurement of transit time through bone alone. A data bank enables his evaluation of the meaning of variations in measurements of transit time, which the inventor deduces to be propagation velocity through each measured bone. The inventor's U.S. Pat. No. 4,913,157 operates on the same general principle of transit-time/velocity deduction, using the later preferred frequency of 2.25 MHz as the base frequency of pulsed "launchings", and he purports to derive the bone-transfer function from analysis of an average of received pulses. In his U.S. Pat. No. 4,941,474, the inventor further refines his technique of transit-time/velocity deduction, inter alia, by separately determining the ratio of the velocity of his observed "bone signal" to the velocity of his observed "soft-tissue signal", using the technique of matched filtering/Fourier transform filtering set forth in his U.S. Pat. No. 4,913,157.
Palmer et al. U.S. Pat. No. 4,774,959 discloses apparatus for deriving the slope of the relation between ultrasonic frequency and attenuation, for the case of a sequence of tone signals, in the range 200 to 600 kHz, applied to one transducer and received by another transducer, (a) after passage through a heel bone, in comparison with (b) passage between the same two transducers without the intervening presence of the heel. The assumption necessarily is that the frequency/attenuation relation is a straight line, i.e. of constant slope.
Brandenburger U.S. Pat. No. 4,926,870 discloses another in vivo bone-analysis system which depends upon measuring transit time for an ultrasonic signal along a desired path through a bone. A "Canonical" wave form, determined by previous experience to be on the correct path, is used for comparison against received signals for transmission through the patient's bone, while the patient's bone is reoriented until the received signal indicates that the patient's bone is aligned with the desired path. Again, ultrasonic velocity through the patient's bone is assumed to have been determined from measured transit time.
Rossman et al. U.S. Pat. No, 5,054,490 discloses an ultrasound densitometer for measuring physical properties and integrity of a bone, upon determination of transit time, in vivo, through a given bone, in comparison with transit time through a medium of known acoustic properties; alternatively, the Rossman et al. device compares absolute attenuation of specific frequency components of ultrasound acoustic signals through the bone with the absolute attenuation of the same frequency components through a medium of known acoustic properties. For attenuation measurements, a "broad-band ultrasonic pulse" is recommended and is illustrated as a single spike "which resonates with a broadband ultrasonic emission". The necessary comparisons are performed by a microprocessor, resulting in a slope of attenuation versus frequency in the broadband of interest. The frequencies or frequency ranges are not disclosed.
The prior art, exemplified by the references that have been briefly discussed, proceed on the assumptions either that transit time is all-important in assessing acoustic velocity or that only one or a few specific ultrasonic frequencies are significant in the determination of the attenuation versus frequency "slope" of a presumably linear relationship. However, the present inventors have found that the attenuation versus frequency relation for bone is non-linear, over the range of ultrasonic frequencies of likely interest, namely, up to approximately 2 MHz, and that potentially significant data exist and have been discarded or overlooked in the prior art through a preoccupation with measuring transit time and/or the velocity of ultrasonic acoustic propagation through bone and soft tissue. Moreover, prior efforts to achieve a broadband analysis have overlooked a need to assure adequate signal above noise throughout a relevant broadband of ultrasonic frequencies.